This invention relates generally to optical elements, and more particularly, to infrared optical elements.
As is known in the art, optical elements and particularly infrared optical elements such as windows, domes and lenses are often comprised of material which is fabricated from a process known as chemical vapor deposition. The chemical vapor deposition (CVD) process generally includes the steps of directing reactant gases into a reactor vessel disposed at an elevated temperature and chemically reacting said gases to form the material. The material is deposited over a substrate to provide the particular optical element. The CVD process is generally a continuous process in which new reactant gases are introduced into the vessel and by-product gases and undeposited material vapors are vented. Generally, the combination of the reactants and by-product gases as well as the elevated temperatures provide a highly corrosive and possible chemically reducing or oxidizing environment.
It would be desirable in many applications to have optical elements having either layers or gratings buried within the optical element. Such layers or gratings may be used for heating to de-ice the optical element, provide electromagnetic shielding, or provide electromagnetic absorption. These layers may also be used to provide a surface reflective to one or more wavelength ranges of incident electromagnetic energy. Optical elements such as bandpass filters and, dichroic beam splitters which require a pair of surfaces reflectively responsive to different electromagnetic wavelengths, could be fabricated having one or more buried reflective layers.
Nevertheless, as mentioned above, when the optical element is fabricated by providing a chemical vapor deposited material over a reflective and/or conductive surface, the abovementioned high temperature and chemically reducing environment generally will degrade the surface morphology of most of the highly reflective and/or conductive materials. Moreover, when layers of the highly reflective metals such as gold or silver are provided, the high temperature and corrosive environment of the chemical vapor deposition process generally causes the layer to agglomerate. When first deposited, these metals tend to have a mirror-like, smooth and hence reflective surface. However, during chemical vapor deposition, small islands of the material are formed leaving behind holes previously occupied by the material. Moreover, for some materials such as silver, total removal of the layer often occurs. This degradation in the surface morphology leads to reduced conductivity and/or reflectivity of the buried layers. Typically, high conductivity and/or reflectivity are the most important properties of these buried layers. Accordingly, buried layers comprising highly reflective and/or conductive materials are typcially not found within optical elements fabricated from chemically vapor deposited materials.